The present invention relates generally to an electronically controlled carburetor for an internal combustion engine. More particularly, the invention relates to an air/fuel ratio control process in the electronically controlled carburetor which selectively uses either CLOSED LOOP or OPEN LOOP control during engine cranking depending on engine operating conditions.
In the CLOSED LOOP air/fuel ratio control method, an exhaust gas sensor, such as an oxygen sensor, produces a feedback signal determining the duty cycle of a control signal. As is well known, the oxygen sensor signal value is proportional to the oxygen concentration in the exhaust gas assuming that the engine is fully warmed up. On the other hand, if the engine temperature is excessively low, the oxygen sensor signal value will not be proportional to the exhaust gas oxygen concentration. Therefore, as long as the oxygen sensor temperature is below a given temperature, CLOSED LOOP control cannot accurately be performed and, therefore, OPEN LOOP control is carried out.
In some of the air/fuel controls, selection or switching between CLOSED LOOP and OPEN LOOP control is made on the basis of engine coolant temperature. However, in such air/fuel ratio controls, after the engine, and thus the oxygen sensor, is warmed up, the oxygen sensor will cool faster than the engine coolant if the engine is stopped for a while. Therefore, it is possible that the oxygen sensor will operate inaccurately even though the engine coolant temperature is in an acceptable range, upon restarting the engine under warmed condition. In such circumstances, if CLOSED LOOP air/fuel ratio control is carried out, emission control will not be performed accurately.